Fig. 1.

Compartmental model of the placental amino acid transfer system, representing the volumes of the maternal intervillous space (v_m), placental syncytiotrophoblast (v_s), and fetal capillaries (v_f). Amino acid transporters included at the maternal-facing microvillous membrane (MVM) were accumulative transporter (Ac) and exchanger (Ex), and those at the fetal-facing basal membrane (BM) were facilitative transporter (F) and exchanger (Ex). F_m and F_f are the flow inputs (and outputs) in the maternal and fetal compartments respectively. [A] and [B] indicate amino acid concentrations in the various compartments.